1. Field of the Invention
This invention relates generally to the design of cardiac pacemakers and more particularly to dual chamber AV sequential or atrium-synchronous pacemaker.
2. Description of the Related Art
Cardiac pacemakers are medical devices, usually implantable, that can be connected to or that are permanently connected to electrode leads for delivery of electrical stimulations pulses to the tissue (myocardium) of a human heart. Dual chamber pacemakers are capable of generating stimulation pulses for the atrium and the ventricle of a human heart. Biventricular pacemakers usually are capable to stimulate at least three chambers of a human heart that is the right atrium, the right ventricle and the left ventricle.
In a dual chamber pacemaker, this is usually realized by placing electrodes in both the right atrium and right ventricle of the heart.
In a demand-type pacemaker these electrodes are coupled through intravenous and/or epicardial leads to sense amplifiers housed in an implanted pacemaker. Electrical activity occurring in these chambers can thus be sensed. When electrical activity is sensed, the pacemaker assumes that a depolarization following a contraction of the indicated chamber has occurred. If no electrical activity is sensed within a prescribed time interval, typically referred to as an atrial or ventricular escape interval, then a pulse generator, also housed within the pacemaker housing, generates a stimulation pulse that is delivered to the indicated chamber, usually via the same lead as is used for sensing.
Separate stimulation pulse generators are usually provided for each heart chamber (atrium or ventricle) to be stimulated.
A control unit triggers the generation of a respective atrial or ventricular stimulation pulse according to a pre-programmed, variable timing regime in order to provide for adequate timing of the stimulation pulses.
A stimulation pulse to the myocardium may cause a contraction of a respective heart chamber, if the myocardium of that chamber is not in a refractory state and if the stimulation pulse intensity is above the stimulation threshold of the myocardium. A sub-threshold stimulation pulse will not cause a cardiac contraction even if delivered to the myocardium in its non-refractory state.
Depending on the mode of operation, a pacemaker only delivers a stimulation pulse (pacing pulse) to a heart chamber (atrium or ventricle) if needed, that is, if no natural excitation of that chamber occurs. Such mode of operation is called an inhibited or demand mode of operation since the delivery of a stimulation pulse is inhibited if a natural excitation of the heart chamber is sensed within a predetermined time interval (usually called escape interval) so the heart chamber is only stimulated if demanded.
In a demand mode, the pacemakers monitors the heart chamber to be stimulated in order to determine if a cardiac excitation (heartbeat) has naturally occurred. Such natural (non-stimulated) excitation, also referred to as “intrinsic” or “signs” cardiac activity, are manifested by the occurrence of recognizable electrical signals that accompany the depolarization or excitation of a cardiac muscle tissue (myocardium). The depolarization of the myocardium is usually immediately followed by a cardiac contraction. For the purpose of the present application, depolarization and contraction may be considered as simultaneous events and the terms “depolarization” and “contraction” are used herein as synonyms.
In order to monitor the heart chamber and thus to determine whether or not a natural contraction of a heart chamber has occurred a pacemaker has a sensing stage during which operation of the pacemaker is connected to an electrode placed in a respective heart chamber. A natural contraction of a heart chamber can be detected by evaluating electrical potentials sensed by this sensing electrode. In the sensed electrical signal the depolarization of an atrium muscle tissue is manifested by occurrence of a signal known as “P-wave”. Similarly, the depolarization of ventricular muscle tissue is manifested by the occurrence of a signal known as “R-wave”. A P-wave or a R-wave represent an atrial event or a ventricular event, respectively, in the further course of this application.
In a demand mode of operation, the pacemaker monitors the heart for the occurrence of P-waves and/or R-waves. If such signals are sensed within a prescribed time period or time window, which is called atrial or ventricular escape interval, respectively, then the escape interval is reset (i.e., restarted) and generation of a stimulation pulse is inhibited and no unnecessary stimulation pulse is triggered. The escape interval is measured from the last heartbeat, i.e., from the last occurrence of an intrinsic (sensed) atrial event (P-wave, A-sense, AS) if the atrium is monitored, or an intrinsic (sensed) ventricular event (R-wave, V-sense, VS) if the ventricle is monitored, or the generation of a stimulation pulse (V-pace, VP; A-pace, AP) if no respective intrinsic event has occurred. If the escape interval “times-out”, i.e., if a time period equal to the escape interval has elapsed without the sensing of a P-wave and/or R-wave (depending upon which chamber of the heart is being monitored), then a stimulation pulse is generated at the conclusion of the escape interval, and the escape interval is reset, i.e., restarted. In this way, the pacemaker provides stimulation pulses “on demand,” i.e., only as needed, when intrinsic cardiac activity does not occur within the prescribed escape interval.
Several modes of operation are available in a state of the art multi mode pacemaker. The pacing modes of a pacemaker, both single and dual or more chamber pacemakers, are classified by type according to a three letter code. In such code, the first letter identifies the chamber of the heart that is paced (i.e., that chamber where a stimulation pulse is delivered), with a “V” indicating the ventricle, an “A” indicating the atrium, and a “D” indicating both the atrium and ventricle. The second letter of the code identifies the chamber wherein cardiac activity is sensed, using the same letters, and wherein an “O” indicates no sensing occurs. The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting (“I”) response wherein a stimulation pulse is delivered to the designated chamber at the conclusion of the appropriate escape interval unless cardiac activity is sensed during the escape interval, in which case the stimulation pulse is inhibited; (2) a Trigger (“T”) response wherein a stimulation pulse to a prescribed chamber of the heart a prescribed period of time after a sensed event; or (3) a Dual (“D”) response wherein both the Inhibiting mode and Trigger mode may be evoked, e.g., with the “inhibiting” occurring in one chamber of the heart and the “triggering” in the other.
To such three letter code, a fourth letter “R” may be added to designate a rate-responsive pacemaker and/or whether the rate-responsive features of such a rate-responsive pacemaker are enabled (“O” typically being used to designate that rate-responsive operation has been disabled). A rate-responsive pacemaker is one wherein a specified parameter or combination of parameters, such as physical activity, the amount of oxygen in the blood, the temperature of the blood, etc., is sensed with an appropriate sensor and is used as a physiological indicator of what the pacing rate should be. When enabled, such rate-responsive pacemaker thus provides stimulation pulses that best meet the physiological demands of the patient.
Multiple-mode, demand-type, cardiac pacemakers allow a sequence of contractions of the heart's chamber which equals as far as possible a natural behavior of the healthy heart for damaged or diseased hearts that are unable to do so on their own.
In a healthy heart, initiation of the cardiac cycle normally begins with depolarization of the sinoatrial (SA) node. This specialized structure is located in the upper portion of the right atrium wall and acts as a natural “pacemaker” of the heart. In a normal cardiac cycle and in response to the initiating SA depolarization, the atrium contracts and forces the blood that has accumulated therein into the ventricle. The natural stimulus causing the atrium to contract is conducted to ventricle via the atrioventricular node (AV node) with a short, natural delay, the atrioventricular delay (AV-delay). Thus a short time after an atrial contraction (a time sufficient to allow the bulk of the blood in the atrium to flow through the one-way valve into the ventricle), the ventricle contracts, forcing the blood out of the ventricle to body tissue. A typical time interval between contraction of the atrium and contraction of the ventricle might be 180 ms; a typical time interval between contraction of the ventricle and the next contraction of the atrium might be 800 ms. Thus, in a healthy heart providing proper AV-synchrony an atrial contraction (A) is followed a relatively short time thereafter by a ventricle contraction (V), that in turn is followed a relatively long time thereafter by the next atrial contraction and so on. Where AV synchrony exists, the heart functions very efficiently as a pump in delivering life-sustaining blood to body tissue; where AV synchrony is absent, the heart functions as an inefficient pump.
To mimic the natural behavior of a heart, a dual-chamber pacemaker, in conventional manner, defines a basic atrial escape interval (AEI) that sets the time interval for scheduling an atrial stimulation pulse. The atrial escape interval can be started by a ventricular event and end with an atrial event. A basic AV delay (AVD) or ventricular escape interval (VEI) sets the time interval or delay between an atrial event and a ventricular event. In such embodiment, AEI and AVD (or VEI) thus together define a length of a heart cycle which is reciprocal to the pacing rate at which stimulation pulses are generated and delivered to a patient's heart in the absence of sensed natural cardiac activity.
For the purpose of this application, a “ventricular event” may refer either a natural ventricular excitation (intrinsic ventricular event) which is sensed as an R-wave or a ventricular stimulation pulse (V-pulse, VP). Similarly, an atrial event shall refer to both, a P-wave or an atrial stimulation pulse (A-pulse, AP).
Since the atrial escape interval usually defines the time of delivery of a next scheduled atrial stimulation pulse, and since an atrial stimulation pulse may be timed from the latest ventricular event as well as from the latest atrial event, in some cases the atrial escape interval is an A-A interval.
In general, two kinds of dual-chamber DDD-timing schemes are known:
In atrial-based DDD pacing, all timing is controlled either from the sensing of atrial activity (a P-wave) or an atrial pacing. When a P-wave is sensed, two separate timers are started that operate in parallel. A first timer defines an atrial escape interval, which, if timed-out, results in an atrial paced event. A second timer defines a separate AV delay, which, if timed-out, results in a ventricular paced event. The first and second timers both start upon sensed or paced atrial activity. The AV delay timer does not affect the basic atrial escape interval timer. The atrial escape interval timer thus controls the basic functioning rate of the pacemaker from atrial to atrial event. The ventricle is paced, if needed, at a rate that tracks the sensed atrial rate. If no atrial activity is sensed, then the atrium is also paced at a rate equal to the set rate. Nonetheless, even when operating in such atrial-based mode, there still remains a need to enhance pacemaker longevity, as well as a need to allow the heart to beat at its own rhythm more often.
In atrial based DDD-pacing the atrial escape interval usually is an A-A-interval which is simultaneously started with a ventricular escape interval (AV-interval)
As an alternative to above-described atrial-based DDD pacing, there is also a second type of dual chamber operation known as ventricular-based pacing. In ventricular-based DDD pacing (sometimes referred to as ventricular-based timing), two parallel timers are used, as indicated above. By a ventricular event, a VA Delay timer is started. If the VA Delay timer times-out all the way, an atrial pulse (A-pulse) is provided. Thus, the VA-delay timer defines an atrial escape interval (AEI). If a P-wave is sensed before the VA Delay timer times-out, such sensing terminates the VA Delay timer. The sensing of a P-wave or the generating of an A-pulse thus define a atrial event. A ventricular event also starts a Ventricular Escape Interval timer. If this Ventricular Escape Interval timer times-out all the way, a ventricular pulse (V-pulse) is provided. If an R-wave is sensed before the Ventricular Escape Interval timer times-out, such sensing terminates the Ventricular Escape Interval Timer. The sensing of an R-wave or the generating of a V-pulse thus comprise a ventricular event, which ventricular event again starts both, the VA Delay timer and the Ventricular Escape Interval Timer.
In patients having an AV-block, the natural conduction from the atrium to the ventricle is affected. However, the atrium itself may contract in a natural way with a physiologically adequate rate. In the DDD(R) mode of operation an AV-sequential stimulation or atrium-synchronous pacing is possible, which allows to track intrinsic atrial contractions and to stimulate the ventricle with an (artificial) AV delay after each sensed atrial contraction in order to maintain AV synchronicity. In such mode of operation the maximum AV delay between an atrial event and the next paced ventricular event is given by the ventricular escape interval.
The choice of an adequate duration of an escape interval depends at least on two demands: the escape interval shall reflect the natural timing of a healthy heart. Therefore, the ventricular escape interval would be chosen to match the natural atrioventricular conduction time in a healthy heart. On the other hand, it is an object to allow as many natural contractions of a heart chamber as possible. Therefore, timeout of the escape interval should not occur to early to give the heart the chance to contract on its own.
To meet these demands one of the programmable modes that has been used with programmable pacemakers for many years is a mode known as the “hysteresis” mode. The hysteresis mode is used in conjunction with selected other modes, such as single-chamber demand pacing, to allow the natural sinus rhythm of the heart to persist at rates less than the programmed minimum rate of the pacemaker. The programmed minimum rate of the pacemaker, in turn, sets the atrial escape interval. During pacing, i.e., during those times when the pacemaker is generating stimulation pulses, the pacemaker thus stimulates the heart at the rate set by the atrial escape interval or the sum of the atrial escape interval and the AV delay, respectively, i.e., upon the timing-out of each atrial and/or ventricular escape interval. When the hysteresis mode is enabled, sensed cardiac activity causes the pacemaker escape interval to be extended, or lengthened, thereby providing a longer period of time within which natural cardiac activity may occur before the pacemaker steps in to generate a stimulation pulse. Should the intrinsic rate of the heart fall below the programmed hysteresis rate, i.e., should no intrinsic cardiac activity be sensed during the lengthened escape interval, then a stimulation pulse is generated, and the escape interval reverts back to its initial value, as determined by the programmed minimum rate.
Further intervals set to determine the pacemaker's behavior include refractory periods like a post ventricular refractory period (PVARP), which is started with delivery of a ventricular pacing pulse and during which no atrial activity is sensed thus rendering the pacemaker refractory (insensitive) in the atrium during PVARP. This interval and other intervals will not be discussed further herein since they are known to one skilled in the art.
State of the art pacemakers are disclosed for example in U.S. Pat. No. 4,856,523, U.S. Pat. No. 5,237,992 and U.S. Pat. No. 5,374,281.
It is an object of the present invention to reduce the percentage of ventricular pacing by promoting the intrinsic AV conduction, while maintaining the ability to switch to DDD(R) in the event of unacceptable AV conduction in particular with patients suffering from a sick sinus syndrome (SSS) and AV block I/II.
Dual chamber pacemakers are often implanted in patients with Sick Sinus Syndrome (SSS). Although such patients often have intact AV conduction or 1st degree AV block (AV-block I), the device is often programmed to the DDD(R) mode with factory AV delay setting, which is more suitable to third degree AV block (AV-block III) patients. Therefore, many patients are unnecessarily paced in the ventricle.
There is growing medical evidence that inappropriate ventricular pacing has disadvantageous short-term hemodynamic effects, and may be less than desirable after an extended period of time, e.g. the Mode Selection Trial (MOST) demonstrated that ventricular desynchronization imposed by ventricular pacing even when AV synchrony is preserved increases the risk of heart failure hospitalization and incidence of AF in 2010 SSS patients with normal QRS duration. A 10% increase in cumulative ventricular pacing was associated with a 20% increased risk of CHF hospitalization. This risk increases linearly until cumulative ventricular paced time reached 60% and then plateaus.